This randomized phase II/III trial studies how well paclitaxel, carboplatin, and metformin hydrochloride works and compares it to paclitaxel, carboplatin, and placebo in treating patients with endometrial cancer that is stage III, IV, or has come back. Drugs used in chemotherapy, such as paclitaxel and carboplatin, work in different ways to stop the growth of tumor cells, either by killing the cells, by stopping them from dividing, or by stopping them from spreading. Metformin hydrochloride may help paclitaxel and carboplatin work better by making cancer cells more sensitive to the drugs. It is not yet known whether paclitaxel and carboplatin is more effective with or without metformin hydrochloride in treating endometrial cancer.